The invention relates to electromagnetic scanners of the type positionable opposite a moving ferromagnetic surface of a textile machine, such scanner being adapted to generate, by induction, an electrical output signal which varies with the rate of movement of the regularly grooved portions of the surface past the scanner in a direction crossing the grooves.
In a known arrangement of this type, a first AC-excited coil supported in magnetic coupling relation to a core is adapted to sense the change in reluctance sensed by the core as the alternating teeth and grooves of the ferromagnetic surface move past the scanner. A second coil is also disposed in magnetic coupling relation to the first coil to generate a voltage proportional to the changes in magnetic excitation of the first coil, and the voltage generated by the second coil is processed in an electronic evaluation circuit, with the results of such evaluation being arranged to control the parameters of the textile machine associated with the moving ferromagnetic surface.
A disadvantage of such arrangement is the relatively low amplitude of induced voltage in the second coil, particularly in the case of a large number of closely spaced grooves on the ferromagnetic surface. In addition, the electronic evaluation circuits for processing the induced voltage have been complicated and expensive.